1. Field of the Invention
The present invention relates to a feeding device that is included in an image forming apparatus, such as a copying machine, a facsimile, and a printer. More particularly, the present invention relates to a feeding device that causes a conveying member to suck the top sheet material out of sheet materials which are stacked in a sheet material housing and feeds the sucked sheet material to the following process, and an image forming apparatus including the feeding device.
2. Description of the Related Art
In a feeding device that is included in an image forming apparatus or the like, it is important to send a plurality of sheet materials such as recording sheets stacked in a sheet material housing accurately one by one. Therefore, the feeding device needs to include a separating mechanism for separating the stacked sheet materials one by one. A widely used separating mechanism is a friction separation type separating mechanism that separates and feeds sheet materials which are sent from the sheet material housing by a pick-up roller, based on a frictional force. The combination of a separating roller and a friction pad, the combination of a separating roller and a reverse roller, or the like is used as a friction separation type separating mechanism.
As a feeding device that includes a separating mechanism different from the friction separation type separating mechanism, Japanese Patent Application Laid-open No. 2007-45630 and Japanese Patent Application Laid-open No. 07-101575 disclose a feeding device that separates and feeds sheet materials by causing a conveying member to suck the sheet material thereto by generating negative pressure. There will be described below the outline of an example of the feeding device which separates and feeds sheet materials by causing a conveying member to suck the sheet material thereto.
A feeding device 200 shown in FIG. 11 includes a sheet-feed tray 6 and a suction belt 2. The sheet-feed tray 6 is a sheet material housing in which a plurality of sheets P, sheet materials, are stacked in a substantially horizontal manner. On the other hand, the suction belt 2 is a conveying member and is provided with an upper surface suction unit 40 inside thereof.
When a command to start feeding of a sheet is issued against the feeding device 200 from a control unit of an image forming apparatus (not shown), blowing units 1 start to blow air and the upper surface suction unit 40 starts to suck air while driving of the suction belt 2 is stopped as shown in FIG. 12. When the blowing unit 1 start to blow air, air is blown to the front end portion and the side end portion of each of the sheets P as shown by arrows A1 and A2. Accordingly, air is introduced between the sheets P, so that a top sheet P1 of the stacked sheets P floats. When the upper surface suction unit 40 starts to suck air, negative pressure is produced as shown by an arrow B of FIG. 11, so that the floating top sheet P1 is sucked onto the suction belt 2.
When a predetermined time (for example, three seconds) has passed since starting of the blowing operation by the blowing units 1 and the suction operation by the upper surface suction unit 40, the suction belt 2 and a pair of conveying rollers 8 start to operate while the blowing units 1 and the upper surface suction unit 40 are operating as shown in FIG. 13. The suction belt 2 is driven and thus the surface of the suction belt moves in a direction of an arrow C in FIG. 13, so that the top sheet P1 sucked onto the lower surface of the suction belt 2 is conveyed to the downstream side in the sheet feeding direction and reaches the pair of conveying rollers 8. After that, the pair of conveying rollers 8 is rotated in the directions of arrows G in FIG. 13, so that the top sheet P1 is conveyed in a direction of an arrow D in FIG. 11 and reaches an image forming section corresponding to the following process. Then, an image is formed on the top sheet.
When the front end of the top sheet P1, which is conveyed by the suction belt 2 and the pair of conveying rollers 8, is detected by a sheet feed sensor 9 as shown in FIG. 14, the driving of the suction belt 2 is stopped. When the driving of the suction belt 2 is stopped in the state in which the upper surface suction unit 40 is operating, a force for stopping conveyance is applied to a portion of the top sheet P1 sucked onto the suction belt 2. However, the material, the nip pressure, or the like of the pair of conveying rollers 8 is set in the feeding device 200 so that a conveying force which is applied to the sheet P by the pair of conveying rollers 8 is sufficiently larger than the force for stopping conveyance. For this reason, even though the driving of the suction belt 2 is stopped, the top sheet P1 continues to be conveyed by the pair of conveying rollers 8.
Here, a sheet, which is stacked under the top sheet P1 of the sheets P, is referred to as the next top sheet P2. While the top sheet P1 is sucked onto the suction belt 2 as shown in FIGS. 12 to 14, the front end portion of the next top sheet P2 is flapped below the top sheet P1 due to the air blown from the blowing units 1. Accordingly, the front end of the next top sheet P2 is separated from the sheet P that is positioned below the next top sheet.
Subsequently, immediately after the rear end portion of the top sheet P1 passes through the suction area formed by the upper surface suction unit 40, the next top sheet P2 is caused to float by the flow of air, which is formed between the upper surface suction unit 40 and the blowing units 1, as shown in FIG. 15, and is sucked onto the suction belt 2.
Then, after a predetermined time has passed since the sheet feed sensor 9 detects the front end of the top sheet P1 shown in FIG. 14, the suction belt 2 starts to be driven again according to a sheet feeding interval which is set. Accordingly, in a similar manner to the top sheet P1 shown in FIG. 13, the next top sheet P2 is conveyed to the downstream side in the sheet feeding direction by the suction belt 2, reaches the pair of conveying rollers 8, and is further conveyed to the further downstream side by the pair of conveying rollers 8.
After that, if the on/off control of the driving of the suction belt 2 is performed while the blowing units 1, the upper surface suction unit 40, and the pair of conveying rollers 8 are operating, the operations of FIGS. 13 to 15 are repeated and the sheets P are sequentially fed to the image forming section one by one.
The blowing unit 1 not only makes the top sheet P1 float but also separates the front end portions of the sheets P from each other by flowing air into a gap between the sheets P. Since the top sheet P1 of the sheets P of which the front end portions are separated from each other is sucked onto the suction belt 2 and conveyed, it may be possible to separate the top sheet P1 from the other sheets P and to feed only the top sheet P1.
Further, the feeding device 200 includes a sheet upper surface sensor 3, which detects the height of the upper surface of the top sheet P1, to maintain a distance h between the lower surface of the suction belt 2 and the upper surface of the top sheet P1 of the sheets P, which are stacked in the sheet-feed tray 6 and decrease in number due to the feeding of the sheets, within a constant range. A lifting mechanism 150 which moves a bottom plate 61 of the sheet-feed tray 6 up and down is controlled on the basis of the detection signal from the sheet upper surface sensor 3. Accordingly, the height of the bottom plate 61 is adjusted, and the distance h between the lower surface of the suction belt 2 and the upper surface of the top sheet P1 of the sheets P stacked on the bottom plate 61 is controlled so as to be kept within a constant range.
The pair of conveying rollers 8 is disposed on the downstream side of the suction belt 2 in the sheet feeding direction, and further conveys the sheet P, which has been conveyed by the suction belt 2 and reached the two conveying rollers, to the further downstream side. Moreover, the sheet feed sensor 9, which detects the passing of the sheet P, is provided on the downstream side of the pair of conveying rollers 8 in the sheet feeding direction.
In a feeding device which separates and feeds sheets P while a sheet P is sucked onto a suction belt 2, like the above-mentioned feeding device 200, the sheets might not be sufficiently separated from each other under a certain ambient humidity around the feeding device or a certain condition of sheets P.
In detail, if the ambient humidity of the feeding device is high and sheets are apt to adhere to each other due to moisture contained in the sheets, the top sheet P1 and the next top sheet P2 adhere to each other. If the sheets P adhere to each other, air cannot be injected into a gap between the top sheet P1 and the next top sheet P2 even though air is blown to the front end portions of the sheets, and air is likely to be injected into the gap between the next top sheet P2 and the sheet P positioned below the next top sheet. In this state, the top sheet P1 and the next top sheet P2 are not separated from each other and thus are sucked together onto the suction belt 2 in an overlapping manner.
Further, when the sheets P are in a certain state, that is, in a state in which the front ends of sheets curl down, even though air is blown to the front end portion of the sheets, it is difficult to inject air into the gap between the top sheet P1 and the next top sheet P2. For this reason, air may not be injected into the gap between the top sheet P1 and the next top sheet P2 but, instead, it is likely to be injected into the gap between the next top sheet P2 and the sheet P positioned below the next top sheet. In this state, the top sheet P1 and the next top sheet P2 are not separated from each other and thus are sucked together onto the suction belt 2 in an overlapping manner.
If the suction belt 2 starts to be driven while the top sheet P1 and the next top sheet P2 overlap each other and are sucked together onto the suction belt 2, two sheets are conveyed in an overlapping state. As a result, double feeding occurs.
If double feeding occurs, sheets P are unnecessarily consumed or blank sheets are mixed in printouts so that the quality of a feeding device significantly deteriorates.
In the feeding device according to an aspect of the invention, a suction force of the lower suction unit which is exerted on the top sheet material is smaller than a suction force that is exerted on the top sheet material by the upper surface suction unit. Accordingly, while the upper surface suction unit and the lower suction unit are operating, the top sheet material is sucked onto the conveying member. Here, when the next top sheet material, which becomes a top portion of the plurality of sheet materials after the top sheet material is fed, adheres to the top sheet material and thus both of the top sheet material and the next top sheet material are likely to be sucked together onto the conveying member; the suction force of the lower suction unit is exerted on the next top sheet material positioned below the top sheet material. In this case, since the suction force of the upper surface suction unit is exerted only on the top sheet material and is not likely to be exerted on the next top sheet material positioned below the top sheet material; the next top sheet material positioned below the top sheet material is applied with only the suction force of the lower suction unit. Since the suction force of the lower suction unit is larger than the adhesion between the sheet materials, the next top sheet material overlapping the top sheet material is lifted down by the suction force of the lower suction unit and only the top sheet material is sucked onto the conveying member. Accordingly, it may be possible to more reliably feed only the top sheet material.